Generally, a transformer is a device that changes AC voltage or DC voltage using the effect of electro-magnetic induction. A power transformer connected to a usual power transmission and distribution line comprises a primary winding connected to a power supply and a secondary winding connected to a load, and changes a voltage to a higher or lower voltage without changing a power.
And, various researches have been conducted on the development of the superconducting power device without resistance loss, since a superconductor had been developed. The development of the high temperature superconductor (HTS) overcoming the limitation of the low temperature superconductor which can work at an extremely low temperature has accelerated the research for the practical use of the superconducting power device, and the research for the practical use of the superconducting transformer using a superconductor (hereinafter, “the superconducting power transforming apparatus”) has been continued.
The superconducting power transforming apparatus has the merit of being small, light, highly efficient, environment-friendly and overload-resistant by comparison with usual transformer.
At an early stage of researches about the superconducting power transforming apparatus, it was recognized as a principal merit of the superconducting power transforming apparatus to enable to save energy and reduce costs for operation by high efficiency.
However, recently, it is recognized as more principal merit to enable to reduce its weight and size.
A standard capacity of 3-phase 154 kV class transformer in general use is 60MVA, and that is installed and operated at a basement of a building in large cities.
As a load is increasing in the near future, the size of the existing transformer using a cooper cable will increase as a consequence of the capacity increase of that. The size increase of a transformer will raise the problem that the existing transformer should be moved to more wide space.
Here, if the superconducting transformer will be substitute for the existing transformer, the limitation of the installation space corresponding to the capacity increase will be solved through reducing its size by half or one third than the existing transformer.
Meanwhile, it is ideal that a secondary voltage (load voltage or low tension) is maintained at a rated voltage in a transformer. But, because a primary voltage is irregular at each installation place and the internal voltage drop of a transformer is changed by the load current intensity and the power factor, the problem occurs that a secondary output voltage is fluctuated too. Therefore, on-load tap changer (OLTC) which adjusts the secondary voltage to the rated voltage by means of the change of turns-ratio is installed and used in the transformer so as to adjust the secondary voltage to the rated voltage for the changing load.
The adjustment of the secondary voltage to the rated voltage is also necessary for the superconducting transformer. Therefore, on-load tap changer (OLTC) which may be applied to the superconducting transformer have to be considered for a practical use of the superconducting transformer.
Generally, the existing transformer and on-load tap changer are installed and operated together while they are immersed in the insulating oil.
However, even though the superconducting transformer is put to practical use, the problem occurs that the installation method of the existing transformer and on-load tap changer can't be applied to the superconducting transformer. Namely, the “high” of the high temperature superconducting transformer just means higher temperature than “low” temperature of the low temperature superconducting transformer. Because the high temperature superconducting transformer even works at extremely low temperature which is much lower than the existing transformer does, the high temperature superconducting transformer can't works in a state that it is immersed in the insulating oil with on-load tap changer together.